Subscriber line interface circuits are typically found in the central office exchange of a telecommunications network. A subscriber line interface circuit (SLIC) provides a communications interface between the digital switching network of a central office and an analog subscriber line. The analog subscriber line connects to a subscriber station or telephone instrument at a location remote from the central office exchange.
The analog subscriber line and subscriber equipment form a subscriber loop. The interface requirements of an SLIC result in the need to provide relatively high voltages and currents for control signaling with respect to the subscriber equipment on the subscriber loop. Voiceband communications are low voltage analog signals on the subscriber loop. Thus the SLIC must detect and transform low voltage analog signals into digital data for transmitting communications received from the subscriber equipment to the digital network. For bi-directional communication, the SLIC must also transform digital data received from the digital network into low voltage analog signals for transmission on the subscriber loop to the subscriber equipment. Strict gain and longitudinal balance control are required for subscriber loop applications.
Multiple high voltage operational amplifiers are frequently used to drive the subscriber loop. The battery feed potential is often used as the negative supply voltage for the operational amplifiers. The operational amplifiers drive the tip and ring voltages of the subscriber loop at levels ranging from ground potential to the battery feed potential, during battery feed and subscriber loop ringing.
One disadvantage of the use of high voltage operational amplifiers is that the operational amplifier bias currents result in non-negligible power losses due to the large potential difference between the positive power supply potential and the battery feed potential. The operational amplifiers are also custom integrated circuits manufactured in a “non-mainstream” high voltage bipolar technology. Yet another disadvantage of this approach is that the operational amplifiers must be closely matched in performance characteristics over a large operating range to ensure proper performance.